/Engine/Source/Runtime/Engine/Classes/Engine/TextureLODSettings.h
Calculate num optional mips
Num optional mips counted from higest mip
int32 CalculateNumOptionalMips ( int32 LODGroup, const int32 Width, const int32 Height, const int32 NumMips, const int32 MinMipToInline, TextureMipGenSettings InMipGenSetting ) const